This requires part 1 (which can take hours to run but generally
doesn't require downtime) to be completed first.
This portion of the migration will require the server to be completely
down for a brief period; for chat.zulip.org with 250M UserMessage
rows, it took about 60s to run; that time will vary depending on
hardware details like whether the server has an SSD, but fundamentally
shouldn't be long.
Our upgrade-zulip and upgrade-zulip-from-git tools can apply this
migration correctly; nothing special needs to be done.
Fixes#13040.
Mateusz Mandera